[iOS] Add DisableU18FeedbackIos feature flag

This flag is created to disable feedback for U18. A similar flag
will be created for desktop.
Since desktop will have a different flag (with 3 values), so it is
relevant to have a different flags on desktop.

Bug: 485542196
Change-Id: I145ebd77e8210f7b4fa322d91feef123d5a033cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7602634
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Tomek Jurkiewicz <tju@google.com>
Commit-Queue: Tomek Jurkiewicz <tju@google.com>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1589367}